为什么 javascript 中的“假”为真？

Question

I understand that an empty string is falsy in javascript and a not-empty string is truthy in javascript.

However, why is 'false' truthy in javascript, is there anything explicit in the specification? Is it a performance issue or are there situations where you would want the string 'false' to represent true ?

Answer 1

is there anything explicit in the specification?

Yes :

The result is false if the argument is the empty String (its length is zero); otherwise the result is true.

Answer 2

Replying to the last part of your question:

Are there situations where you would want the string 'false' to represent true?

Let's consider I am testing for empty strings in my user input. To achieve that, I issue:

if (!theInput) {
    // Do something.
}

Now do I want that condition to be true if the user enters false in the text box? Of course, I don't.

Answer 3

In Javascript any string that is not empty is truthy.

So, when evaluating any non-empty string results in true even when the string itself is 'false' .

You can read more about truthy and falsy values.

If you want to check a string for truthness, you can check it's length.

var val = str.length > 0;

Answer 4

The value of a non-empty string is always true.

Boolean(false) returns false

Boolean('false') returns true

Answer 5

It's by definition. It's a string and it is handled as such. No matter the meaning of the string.

If your logic would be applied. Then how about following examples:

"1+3==2"
"humans have four eyes"

Are they also falsy?

Answer 6

If you consider the Boolean values, false is false and true is true.

Boolean(true) //returns true
Boolean(false) //returns false

But when you are talking about the strings like 'true' and 'false' or any other non-empty string, Javascript will not read them as Booleans and they will be true as all other non-empty strings.

Boolean('true') //returns true
Boolean('false') //returns true
Boolean('blahblahblah') //returns true